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I have a laptop with an external monitor connected via HDMI (output name is HDMI1). The command xbacklight works only with my primary screen (which is the laptop screen).

It doesn't work even if I try running it with argument -display HDMI1 (which gives me an error RANDR Query Version returned error -1) or --display HDMI1 (which gives me the help information).

I have seen this - Turn off the backlight of an external monitor - question but it only mentions how to change the brightness, not backlight.

EDIT: the option -display is not documented in the manual page of xbacklight, but after a quick look into the source code I've found where it's used. In the function xcb_connect it represents the X display, not the actual screen name.

1

You can do it with ddcutil, as mentioned here, but it's not that responsive and requires root privileges:

# ddcutil capabilities | grep Brightness
  Feature: 10 (Brightness)

# ddcutil getvcp 10
VCP code 0x10 (Brightness                    ): current value =    60, max value =   100

# ddcutil setvcp 10 70

I don't think that there's "one size fits all" solution since the backlight configuration depends on the hardware. In some situations, you cannot control the backlight even of the built-in laptop monitor.

  • All I'm getting is Display not found, even after adding -d flag. ): – Jezor Jun 4 '18 at 18:43
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xbacklight may be your answer:

https://wiki.archlinux.org/index.php/backlight#xbacklight

Brightness and Backlight seems to be the same thing. If so, go for xrandr since every output are supported.

  • 1
    These two are NOT the same thing. Brightness in xrandr represents pixels "whiteness", backlight in xbacklight represents the LED backlight. I took a picture of both of my screens - i.imgur.com/oSXfCGr.jpg The left screen has brightness set to 0 with xrandr, the right one has backlight set to 0 with xbacklight. Notice the "glare" - these are pixels displaying black color and being shined on. (; – Jezor Feb 2 '18 at 0:48
  • See also the wikipedia article: en.wikipedia.org/wiki/Backlight – Jezor Feb 2 '18 at 0:49

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